This invention is directed at a method of sensing flow and providing a switch to indicate the presence of flow of a fluid that is at very low temperature (xe2x88x9240 C to xe2x88x92273 C). More particularly this invention is intended to sense the flow of a refrigerant at these very low temperatures.
Refrigeration systems have been in existence since the early 1900s, when reliable sealed refrigeration systems were developed. Since that time, improvements in refrigeration technology have proven their utility in both residential and industrial applications. In particular, very low-temperature refrigeration systems currently provide essential industrial functions in biomedical applications, cryoelectronics, physical vapor deposition vacuum coating processes, and semiconductor manufacturing applications.
This invention relates to refrigeration systems which provide refrigeration (removal of heat) at temperatures between xe2x88x9240 C and xe2x88x92273 C. The temperatures encompassed in this range are variously referred to as low, ultra low and cryogenic. For purposes of this Patent the term xe2x80x9cvery lowxe2x80x9d or very low temperature will be used to mean the temperature range of xe2x88x9240 C to xe2x88x92273 C.
As a customer security measure, closed loop, very low temperature refrigeration systems must provide a refrigerant flow indicator in highly sensitive applications. In general, these systems require that all components withstand operating pressures between 1 and 31 atmospheres, and maintain a leak tight integrity.
Traditional means of sensing refrigerant flow requires moving elements, such as turbine meters, or other means of displacement of the sensing element. In refrigeration systems, small amounts of compressor oil may be present in the refrigerant that may congeal at low temperatures and interfere with the moving parts of these sensors. Flow sensors or flow switches that meet the leak integrity and working pressure requirements are usually very costly and are susceptible to contamination from this congealed oil. Other means of sensing flow, measure the pressure drop across an element such as an orifice or venturi section and then measure the pressure difference between the two points across or within such an element. These methods require knowledge of the flow properties of the fluid as a function of temperature, pressure and flow rate which can be difficult for applications in which the range of flow conditions vary substantially. The present invention provides a means to sense the presence of flow reliably and without a detailed knowledge of the fluid being sensed.
Many industrial applications require the ability to transfer heat in an efficient manner at a relatively fast rate. In applications where the presence of flow of a cooling media are considered essential to the process, a means of flow indication is required. Examples of such critical applications are storage of biological tissue, or the processing of a silicon wafer as part of a semiconductor manufacturing process. Providing a means of sensing flow or lack of flow alerts the user to the fact that flow is no longer present. In some control applications, where for example a heater is used to control temperature, such a flow indicating device can be used as an interlock to assure that heat is applied only when flow is occurring.
Conventional flow switching devices with displacing elements cannot detect flow of fluids at very low temperature temperatures (xe2x88x9240xc2x0 C. to xe2x88x92273 C.) due to the incompatibility of sensing elements. Such very low temperatures require that the sensing element be suited for operation at very low temperatures. Specifically the materials used must not lose a significant portion of their mechanical strength and must not deform or become brittle. For closed loop refrigeration processes leak tight operation is also mandated. This means that the very low temperature flow switch must also be leak tight. In addition to mechanical strength issues, typical flow switches available have elastomeric seals that leak to the atmosphere at very low temperatures. In addition, the materials used must be chemically compatible with the fluid being sensed. Failure to have proper chemical compatibility results in degradation of sealing material, or moving parts and eventually result in component failure. Flow switches designed to meet all of these various requirements are typically very expensive. The present invention describes a means to provide sensing of a very low temperature fluid while satisfying the requirements for mechanical strength, leak tightness and chemical compatibility. This is achieved using components that are much less expensive than typical flow switches designed to meet these requirements.
There are many vacuum processes which have the need for such very low temperature cooling. The chief use is to provide water vapor cryopumping for vacuum systems. The very low temperature surface captures and holds water vapor molecules at a much higher rate than they are released. The net effect is to quickly and significantly lower the chamber""s water vapor partial pressure. Another application involves thermal radiation shielding. In this application large panels are cooled to very low temperatures. These cooled panels intercept radiant heat from vacuum chamber surfaces and heaters. This can reduce the heat load on surfaces that are being cooled to lower temperatures than the panels. Yet another application is the removal of heat from objects being manufactured. In some cases the object is an aluminum disc for a computer hard drive, a silicon wafer for an integrated circuit, or the material for a flat panel display. In these cases the very low temperature provides a means for removing heat from these objects more rapidly than other means, even though the object""s final temperature at the end of the process step may be higher than room temperature. Further, some applications involving, hard disc drive media, silicon wafers, or flat panel display material, involve the deposition of material onto these objects. In such cases heat is released from the object as a result of the deposition and this heat must be removed while maintaining the object within prescribed temperatures. Cooling a surface like a platen is the typical means of removing heat from such objects. In all these cases it is to be understood that the evaporator surface is where the refrigerant is removing heat from these customer applications when providing cooling at very low temperatures.
Refrigeration systems can also be used to provide a heating function for industrial processes in addition to providing refrigeration. This is needed when the system must provide a defrost or bakeout function. A defrost function delivers warm refrigerant to the object that is typically being cooled. The purpose is to bring the object to a temperature close to room temperature. This is typical of vacuum processes in which the vacuum chamber must be opened to room air. Having the evaporator surface at room temperature eliminates condensation of room air moisture in the vacuum chamber. A bakeout function also delivers warm refrigerant to the evaporator surface that is typically being cooled. Unlike the defrost function the purpose of the bakeout is to warm the evaporator surface above room temperature, typically +60 C to +110 C for an extended period of time, typically 1 to 24 hours. This can be useful when desorbing water vapor from the vacuum chamber after the chamber has been exposed to moist ambient air. During defrost and bakeout processes it is important to verify that refrigerant is flowing. Therefore for the flow switch to be fully effective its design must provide operation at elevated temperatures in addition to very low temperature. The present invention describes a means to satisfy these requirements.
U.S. Pat. No. 5,730,216, xe2x80x9cAir conditioning and refrigeration units utilizing a cryogen,xe2x80x9d assigned to Thermo King Corporation (Minneapolis, Minn.), describes an air conditioning and refrigeration apparatus for controlling the temperature of a conditioned space to a predetermined temperature band adjacent to a predetermined set point temperature via cooling and heating cycles utilizing a supply of pressurized cryogen. A cryogen driven motor drives a fan and an alternator. The alternator provides control voltage for operating electrical control devices which select a heating cycle, or a cooling cycle, as required, and which control the flow of cryogen during the selected cycle. Electrical load on the alternator, and thus a load on the vapor driven motor, is minimized to maintain air delivered by the fan to the conditioned space at a level that provides a substantially uniform air flow and temperature throughout the conditioned space. A cryogen flow control valve is motor controlled and only requires electrical energy when a change in the cryogen flow rate is desired. Heating and cooling cycle selecting valves require no electrical energy during a cooling cycle, and very little electrical energy during a heating cycle, as cryogen pressure is utilized to shift the heating and cooling cycle selecting valves when a heating cycle is desired.
U.S. Pat. No. 4,535,598, xe2x80x9cMethod and control system for verifying sensor operation in a refrigeration system,xe2x80x9d assigned to Carrier Corporation (Syracuse, N.Y.), describes a method and control system for checking and verifying sensor operation in a refrigeration system. A microcomputer control system determines whether a signal provided by the sensor is within normal limits prior to startup of the refrigeration system. Also, the microcomputer system verifies an out of bounds sensor signal provided to the microcomputer system from the sensor during operation of the refrigeration system by intermittently monitoring the sensor to determine whether a pre-selected consecutive number of out of bounds signals are provided to the microcomputer control system from the sensor. If the microcomputer control system determines that the signal provided by the sensor is not within normal limits prior to startup of the refrigeration system, or if the microcomputer system verifies an out of bounds signal provided by the sensor during operation of the refrigeration system, then the microcomputer control system generates an alarm signal to abort the startup or shut down the operation of the refrigeration system, respectively.
One difference between the present invention and the background patents is that the present invention is a combination of conventional elements arranged to detect very low temperature fluid flow in a refrigeration system, whereas the background patents are not.
The present invention utilizes standard industrial components to provide a means of sensing flow of a very low temperature fluid and providing a switch output. This is done while assuring chemical compatibility of the flow switch with the fluid, providing a leak tight design, providing proper design pressures, and providing suitable materials for use at temperatures ranging from a few hundred degrees C to xe2x88x92273 C.
The present invention provides a very low temperature flow switch apparatus for use in a refrigeration system or industrial process. The very low temperature flow switch of the present invention provides a way to detect flow of very low temperature fluid in a refrigeration system or industrial process. The very low temperature flow switch is for use in any type of refrigeration system or process, such as a single-refrigerant system, a mixed-refrigerant system, normal refrigeration processes, cascade refrigeration processes, an auto-refrigerating cascade cycle, a Klimenko cycle, or a single expansion device system. It can also be used for any industrial process that uses very low temperature fluids.
In accordance with the invention, flow in forward direction through a check valve is detected by a signaling device in parallel with the check valve and sensing the pressure differential across the check valve. When the forward differential pressure exceeds the cracking pressure of the valve, the valve opens for a refrigerant flow and substantially concurrently the signaling device outputs an electrical signal to indicate that the valve is open to forward refrigerant flow. A flow restrictor, e.g., a capillary tube, in parallel with the check valve equalizes the refrigerant pressure at the inlet and outlet of the check valve when the differential pressure across the valve drops below the valve cracking pressure and the check valve closes to flow. Whenever the differential pressure is less than cracking pressure, the signaling device changes its electrical output to indicate that the valve is closed to flow. The signaling device may be pressure switches or pressures transducers connected to the valve inlet and outlet respectively or a single pressure differential device in parallel with the control valve.
Sensing the flow of refrigerant and outputting a switch signal provides process verification for the industrial process of interest. In some cases temperature control is achieved by applying heat to refrigerant with a heater. Having a switch that confirms that refrigerant is flowing provides a means to interlock the heater. That is, power to the heater can be interrupted by this switch if fluid flow is not occurring. This is useful to prevent excessive temperatures from occurring when fluid flow is unexpectedly stopped. In applications where continuous flow and temperature stability are important the flow switch provides an early warning that flow has been interrupted.
As an example, biological tissues are typically stored at very low temperatures and are cooled by a very low temperature fluid such as liquid nitrogen or by a refrigerant or a mixture of refrigerants. Having electrical indication that flow has been lost provides a faster indication than waiting for the temperature to begin rising, which can damage the biological tissues being stored. Likewise, any other application needing very low temperature cooling can benefit from verification that flow is occurring.
Still other objects and advantages of the invention will be apparent in the specification.
The invention accordingly comprises the features of construction, combinations of elements, and arrangements of parts, which will be exemplified in the constructions hereinafter set forth, and the scope of the invention will be indicated in the claims.